The use of radiation from lasers in medical and dental procedures has been known for some time, having been applied shortly after the invention of the laser. In early work, which used infrared or visible lasers, medical researchers treated animal and human retinas and showed that the laser beam could induce a lesion on the retina for therapeutic purposes. Such laser eye surgery using visible or infrared lasers for detached retinas and other disorders is now routine in eye clinics throughout the world. In these medical applications, and in other applications using laser beams, the laser beam is absorbed by the irradiated tissue causing heating, denaturing of protein, and tissue death. The results are therapeutic because of the formation of scar tissue, cauterization of blood vessels, or the cutting away of diseased or damaged tissue.
Thus, in the early prior art, lasers were used to provide a directed source of radiation whose thermal energy led to the pyrolysis of the organic matter. However, there are many situations where heating is not desired and is, in fact, harmful; in those situations such lasers may not be used. For example, infrared lasers, which cut by heating a material substrate rather than by photochemical ablation, are normally not desirable for etching polymers and other organic materials (such as biological layers) since the region which is heated cannot be adequately controlled, especially for deep cuts. As will be more apparent from the following, the present invention is directed to a technique and apparatus for etching in a manner which avoids unnecessary heating damage to the substrate. In 1982 it was first reported that ultraviolet radiation of less than 200 nm-wavelength has a very high efficiency for decomposing polymers and organic biological matter by electronic excitation of the constituent bonds of the organic matter, followed by bond breaking. This phenomenon is referred to as ablative photodecomposition (APD). The irradiated material is removed by a ablative photodecomposition without substantially heating or otherwise damaging the remaining material. This is a relatively linear photochemical effect, and inhomogenities in the organic materials do not affect the photo etching. This phenomenon was subsequently found to extend to longer ultraviolet wavelengths. Currently, UV lasers of wavelengths from 193 nm to 351 nm are used in polymer ablation as well as in surgery on the cornea and angioplasty.
Ultraviolet radiation is defined as including wavelengths between 150 and 400 nm. In the art, ablative etching can be accomplished using any known source, as long as the source emits radiation in the wavelength range of less than 400 nm and as long as ablative photodecomposition occurs. One suitable source of ultraviolet wavelength radiation is an ArF excimer laser providing a pulsed output at 193 nm. Such lasers are commercially available.
Ablation is the process by which ultraviolet radiation having wavelengths less than 400 nm is capable of decomposing certain materials by electronically exciting the constituent bonds of the material, followed by bond-breaking and the production of volatile fragment materials which evaporate or escape from the surface. These photochemical reactions are known to be particularly efficient for wavelengths less than 200 nm (i.e., vacuum ultraviolet radiation), although wavelengths up to 400 nm have been used in surgery and other applications. In ablative photodecomposition, the broken fragments of biological matter carry away kinetic energy, thus preventing the energy from generating heat in the substrate.
To etch an organic polymer or biological tissue by ablative photodecomposition, it is necessary that the radiation be absorbed by the medium even at low laser power. However, many materials do not absorb sufficient energy to ensure ablation at low fluence. One possible solution to this problem is to dope these materials with a substance that increases the absorption cross-section of the material. This solution is unacceptable in many applications because doping will contaminate the entire sample to the depth of the desired etch.
It would, therefore, be desirable to etch by ablative photodecomposition using low fluence lasers. In one aspect, this invention relates to a method of etching using a first and second lasers. This combination of ultraviolet laser wavelengths may be used for medical and dental purposes, and more particularly for etching or eroding biological organic material or polymer substrates. Material can be selectively removed without undue heating or damage to the areas surrounding the area struck by the radiation. The technique and apparatus by which the organic material is removed, or etched, is different than that of the prior art, and the geometry of the etching pattern is completely defined by the incident radiation.
Many prior art systems include a second visible laser to aid in aiming a non-visible cutting laser. U.S. Pat. No. 3,710,798 Bredemeier and U.S. Pat. No. 4,289,378 Remy et al. describe laser cutting systems using lasers at two distinct wavelengths. A first laser in the visible spectrum illuminates the target area and a second, cutting laser ablates away the organic material.
U.S. Pat. No. 4,408,602 to Nakajima describes a laser ablation system using three laser sources, the radiation from each source being a distinct wavelength. A first source emits a beam in the visible spectrum to aim the laser while the second and third beams are independent cutting sources. The first of these two cutting sources is a C0.sub.2 laser with a wavelength in the infrared region. The second of the cutting lasers is a "YAG" laser which has a wavelength in the visible spectrum. Each of these lasers is effective on different types of tissue. The apparatus in this patent enables an operator to easily switch between lasers, depending on the tissue he is attempting to cut.
Koren, in his article entitled "CO.sub.2 Laser Assisted UV Ablative Photoetching of Kapton Films," published Jul. 1984 in Applied Physics Letters, describes the use of an infrared laser source to etch a polymer. In this arrangement, a plasma is created by focusing a first portion of the infrared laser radiation on a tungsten rod, creating an extremely high temperature. The continuous spectrum of ultraviolet (UV) and vacuum ultraviolet (VUV) radiation generated by the plasma is focused on the polymer target along with a second portion of the infrared laser radiation, etching the target. This etch technique is not acceptable in many situations since the infrared laser will tend to cause thermal damage to the material being etched. In addition, this technique could not be used where fiber optics conduct and focus the radiation since infrared radiation is almost impossible to conduct through known optical fibers, especially at the fluencies described. Where a substrate is ablated by a single laser, the depth of ablation is a function of the wavelength of the incident radiation, the incident power (fluence) of the laser, and the number and duration of the pulses. Therefore, the etch depth may be controlled by changing any of these variables. However, in many situations, the wavelength (i.e., type of laser) and incident power are fixed by the limitations of the available equipment. In order to accurately control the etch characteristics under these circumstances it would be advantageous to be able to enhance the etch characteristics of the laser, for example, by using a second, longer wavelength laser in time coherence with the etching laser.